I have this exact motor starter on my AC braking motor (FLA 3.5A).

The starting current is 12.1 A however and keeps tripping the relay after approximately ten minutes of use.

The motor operates by raising and lowering a diverter. When raising it turns 180 degrees and likewise when lowering. It raises and stays for 30s then lowers and stays there for 30s.

Why does this motor trip after a while?

Keep in mind the temperature of the motor is 42 degrees C when tripped.

  • \$\begingroup\$ How did you compute that this breaker is a match for your protection? Include actual datasheet of motor and load calculations as well as current for DC braking indicate actual RMS current for load and duration time as well as if 3 phase rectified or unbalanced 1 phase rectified \$\endgroup\$ May 2, 2019 at 13:03

2 Answers 2


Pardon my ignorance on this application, but I do not know the nature of your load current if that was measured or rated. enter image description here

My understanding is the Load Transfer Switch(LTC) & motor operate in a discontinuous mode of AC start-surge then DC-rapid stop, then idle for <30 seconds in order to regulate the primary grid voltage into the transformer. The Hybrid Relay responds to Control Left(Reverse) and Right(Fwd) motor commands in a rotary LTC high-voltage high current-switching Tap Changer for either a Reactive or resistive, On or OFF Load Diverter.

The start/FLA (Full load Amp) current ratio of 3.5 (=12.1A/3.5A) is common for 3ph induction motors. The intervals of start/stop surge-current and the **unknown dwell time* for start/stop, are a major factor in choosing the right protection levels.

This must be a special application for some Load Transfer Switch, either reactive or resistive Load Transfer Changer (LTC) for a large power transformer to regulate the primary coil voltage every 30 seconds. You must be a designer for this power transformer to understand the nature of this load.

Depending on the condition of rotary contacts this motor moves, there may be some characteristic loads I am not familiar with, during the arc-current bypass operation in the load during this motorized transfer-switch operation, during both the motor start and rectified AC to DC winding fast stop operations. The timing and design of these characteristics must be controlled such that LTC motor-drive starts and stops quickly to minimize LTC wear.

If the start current is 12A then a full speed reverse will be 24A and if reversing every 30s your average power will exceed the breaker 9A rating.

  • It is unknown what the Relay current is during a rotate start-stop and if motor uses the DC brake option.

The Hybrid Breaker estimates the heat rise in the motor using some I^2 vs time delay algorithm. - This Current-time profile is also unknown.

  • Since the motor is not too hot when tripped, then the protection level is too low or something has changed in the mechanical loads of LTC contacts that have changed the performance.

It is not a good idea to reverse at full speed as you should stop first then reverse.


I recommend;
- contact the OEM's for motor & LTC load current profiles
- get Phoenix Contact's Amp-second breaker curves
- monitor breaker current characteristics for conditioned maintenance to LTC.
- join / contact Linked-In for a wealth of wisdom in the Transformers group
- consider Consultant: Mr Csanyi for expert advice.

  • 1
    \$\begingroup\$ Even if the motor stops for 30 seconds between raising and lowering, 20 starts in a little more than 10 minutes is likely not what the protection interprets as acceptable. \$\endgroup\$
    – user80875
    May 2, 2019 at 9:15
  • \$\begingroup\$ Motors are usually rated in single digit starts per hour, though it varies depending on the load. \$\endgroup\$ May 8, 2019 at 23:37
  • \$\begingroup\$ I agree @SomeoneSomewhere and this breaker supplies rapid AC Start and DC stop which is more than a typical load. So wrong breaker rating or application interval time. \$\endgroup\$ May 9, 2019 at 2:42

A "brake motor" is usually a motor that has a DC powered brake coil on it that operates to RELEASE a spring powered mechanical brake. In a typical brake motor circuit, the brake coil is powered by the same motor leads that tun the motor. So when you turn on the motor, you are energizing the brake coil simultaneously, releasing the brake. that brake coil is usually DC, because it has to be held in as long as the motor is running, so internally on the motor, there is a little rectifier for creating that DC power from the motor AC circuit.

The little Phoenix "hybrid starter" device shown is apparently (they don't really say) a solid state starter with a bypass contact. That means that when it energizes the motor, it is probably putting out a "chopped" sine wave. That will not function correctly for the brake coil rectifier so it's likely that your brake coil is not fully disengaging, thereby overloading your motor. Did you check with Phoenix if it was OK to use that device with a brake motor? If so, are you sure the person you spoke with understood what they meant? Phoenix is not known for making motor starters, they may not have a clue.


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